Racket

ABSTRACT

A racket including: a handle; an annular frame having a plurality of through holes, and a groove section provided on an outer peripheral face of the frame along a circumferential direction of the frame; a shaft connecting the handle and the frame; and a string that passes through the through hole to the outside of the frame, and that is folded back on the groove section, wherein when either one of a longitudinal direction of the racket and a width direction perpendicular to the longitudinal direction on a hitting surface of the racket is defined as a first direction, a depth of a position at which the string is folded back at a second position in the first direction is smaller or less than a depth of a position at which the string is folded back at a first position in the first direction, the first position being located on the outer peripheral face of the frame, the second position being located on the outer peripheral face of the frame and outside of the first position in the first direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority upon Japanese Patent ApplicationNo. 2013-44426 filed on Mar. 6, 2013 which is herein incorporated byreference.

BACKGROUND

1. Technical Field

The invention relates to a racket used in tennis and the like.

2. Related Art

As a racket used in tennis and the like, there are known ones including:a handle which a player grips; a frame inside which a string is strung;and a shaft connects the handle and the frame. Generally, on the outerperipheral face of the frame, a groove section is provided along thecircumferential direction of the frame and in the middle in thethickness direction (a direction perpendicular to a hitting surface). Inaddition, along the circumferential direction of the frame, the framehas a plurality of through holes through which a string passes. Theopenings of the through holes are formed in the groove section. Whenstringing the string in the frame, the following processes are repeated:the string passes from the inside to the outside of the frame through athrough hole; the string is folded back along the groove section of theframe; and the string passes through the next through hole. Then, in theframe, the portions of the string (longitudinal strings) along thelongitudinal direction of the racket are strung at intervals in thewidth direction intersecting with the longitudinal direction. Also, theportions of the string (transverse strings) along the width directionare strung at intervals in the longitudinal direction so as to intersectthe longitudinal strings. Thus, the net-like hitting surface is formedin the frame (for example, see Japanese Patent Application Laid-openPublication No. 6-269514).

The sweet spot, which has a high rebound performance of a ball andvibrates less when hitting a ball, can be enlarged by increasing thelengths of longitudinal strings and transverse strings and by reducingthe difference between the lengths of longitudinal strings arranged inthe width direction and/or the difference between the lengths oftransverse strings arranged in the longitudinal direction. However, ifthe frame has an annular shape such as an elliptical shape, the stringis folded back along the outer peripheral face (the groove section) ofthe curved frame. Therefore, longitudinal strings and transverse stringspassing the ends of the frame are shorter in length than longitudinalstrings and transverse strings passing the center of the frame.

SUMMARY

The invention has been made in view of the above issue, and an advantagethereof is to enlarge the sweet spot of a racket without changing theshape of the frame.

An aspect of the invention to achieve the above advantage is a racketincluding: a handle; an annular frame having a plurality of throughholes, and a groove section provided on an outer peripheral face of theframe along a circumferential direction of the frame; a shaft connectingthe handle and the frame; and a string that passes through the throughhole to the outside of the frame, and that is folded back on the groovesection, wherein when either one of a longitudinal direction of theracket and a width direction perpendicular to the longitudinal directionon a hitting surface of the racket is defined as a first direction, adepth of a position at which the string is folded back at a secondposition in the first direction is smaller or less than a depth of aposition at which the string is folded back at a first position in thefirst direction, the first position being located on the outerperipheral face of the frame, the second position being located on theouter peripheral face of the frame and outside of the first position inthe first direction.

Other features of this invention will become apparent from thedescription in this specification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view and a side view of a tennis racket.

FIG. 2A is a cross-sectional view of a frame in the thickness direction,and

FIG. 2B is a diagram showing how the string is strung in the frame.

FIG. 3 is a diagram of a groove section provided in the frame of thepresent embodiment.

FIG. 4 is a diagram of a groove section provided in the frame of acomparative example.

FIG. 5 is a diagram of the frame and the groove section of a modifiedexample.

DESCRIPTION OF EMBODIMENTS Overview

From the description in the detailed description of the invention andthe accompanied drawings, at least the following matters will beapparent.

A racket including:

a handle;

an annular frame having

-   -   a plurality of through holes, and    -   a groove section provided on an outer peripheral face of the        frame along a circumferential direction of the frame;

a shaft connecting the handle and the frame; and

a string

-   -   that passes through the through hole to the outside of the        frame, and    -   that is folded back on the groove section,        wherein

when either one of a longitudinal direction of the racket and a widthdirection perpendicular to the longitudinal direction on a hittingsurface of the racket is defined as a first direction,

-   -   a depth of a position at which the string is folded back at a        second position in the first direction is smaller or less than a        depth of a position at which the string is folded back at a        first position in the first direction,    -   the first position being located on the outer peripheral face of        the frame,    -   the second position being located on the outer peripheral face        of the frame and outside of the first position in the first        direction.

With such a racket, it is possible that, without changing the shape ofthe annular frame, the length of the string on any outer end of theframe (the second position) in the first direction becomes more equal tothe length of the string at the center of the frame (the first position)in the first direction. As a result, sweet spot can be enlarged.

In such a racket, the position at which the string is folded back on theouter peripheral face of the frame may become shallower in depth as itgoes from a center of the frame to an end in the first direction.

With such a racket, it is possible to reduce the difference in lengthbetween strings arranged in a direction intersecting with the firstdirection. This makes it possible to reduce the differences caused bydifferent hitting positions in the frame of the rebound performance of aball and the feel, which results in enlarging the sweet spot.

In such a racket, the frame may have substantially an elliptical shape,and a position in an end in the longitudinal direction on the outerperipheral face of the frame at which the string is folded back at acenter in the width direction may be deeper in depth than a position inan end in the width direction on the outer peripheral face of the frameat which the string is folded back at a center in the longitudinaldirection.

With such a racket, the sweet spot can be further enlarged whilepreventing the strength deterioration of the frame. This also allows theracket to rebound a ball more and to vibrate less.

Racket 1

An embodiment of a tennis racket will be described below as an exampleof a racket according to the invention.

FIG. 1 is a plan view and a side view of a tennis racket (hereinafterreferred to as a racket 1). The left one is the plan view and the rightone is a side view. FIG. 2A is a cross-sectional view of a frame 11 inthe thickness direction, and FIG. 2B is a diagram showing how a string12 is strung in the frame 11. The racket 1 includes: a head 10 whichhits a ball; a handle 20 which a player grips to hold the racket 1; anda shaft 30 connects the head 10 and the handle 20 in an integratedmanner. For the purpose of illustration, of the longitudinal sides ofthe racket 1, the side of the head 10 is called as a tip-end side, andthe side of the handle 20 is called as a handle-end side. On the hittingsurface of the racket 1 (that is, on the plane along the hittingsurface), a direction perpendicular to the longitudinal direction iscalled as a width direction. A direction perpendicular to the hittingsurface of the racket 1 is called as a thickness direction.

The head 10 includes: the frame 11 with an elliptical shape extending inthe longitudinal direction of the racket 1; and the string 12 strunginside the frame 11. The frame 11 is made by shaping into an ellipticalshape, for example, a hollow rod which is made of fiber reinforced resinand the like. It should be noted that the frame 11 may not be hollow andthe inside of the frame 11 may be filled with foamed material. As shownin FIG. 2A, in the outer peripheral face 11 a of the frame 11, a groovesection 111 is provided along the circumferential direction of the frame11 and the groove section 111 has the middle in the thickness directionwhich is concave compared to the both side sections 112. In addition,the frame 11 has a plurality of string holes 13 (corresponding tothrough holes) along the circumferential direction of the frame 11; thestring holes 13 extend through from the inner peripheral face 11 b tothe outer peripheral face 11 a and are for inserting the string 12.

As shown in FIG. 2B, the opening of each string hole 13 is provided onthe groove section 111. In order to protect the string 12 and to dampenvibrations caused by hitting a ball, the string 12 is strung in theframe 11 through a grommet 40. The grommet 40 includes: A plurality oftube sections 41; and a strip-shaped base section 42 which connects theplurality of tube sections 41 to each other and has communicating holes43 which communicate the hollow of each tube section 41. The tip end ofeach of the tube section 41 is placed into the string hole 13 from theside of the outer peripheral face 11 a of the frame 11. Then, the basesection 42 is embedded in the groove section 111 of the frame 11. Inthis way, the grommet 40 is placed in the outer peripheral face 11 a ofthe frame 11. In the base section 42 of the grommet 40, a groove section44 extending from one end of the base section 42 to the other end isprovided in the middle of the thickness direction; the openings of thecommunicating holes 43 are provided in the groove section 44. Thethickness of the base section 42 (the thickness from the surface onwhich the tube section 41 is placed to the bottom surface of the groovesection 44) is uniform.

Thus, when stringing the string 12 in the frame 11, the string 12 passesfrom the inside of the frame 11 to the outside through one of the tubesections 41 (the string holes 13) of the grommet 40, and the string 12is folded back on the groove section 44 of the grommet 40. In otherwords, through the groove section 44 of the base section 42 of thegrommet 40, the string 12 is folded back on the groove section 111 ofthe frame 11. Thereafter, the string 12 is inserted into the next tubesection 41 (string hole 13). This process is repeated. Thus, a pluralityof “longitudinal strings 121”, which are the portions of the string 12extending in the longitudinal direction, are strung inside the frame 11at intervals in the width direction. A plurality of “transverse strings122”, which are the portions of the string 12 extending in the widthdirection, are strung at intervals in the longitudinal direction so asto intersect the longitudinal strings 121. Thus, a net-like hittingsurface is formed.

The shaft 30 branches into two sections toward the tip end inlongitudinal direction as seen from the thickness direction, as shown inthe left diagram of FIG. 1. So, an opening 31 extending in the thicknessdirection is formed by the branched portions and the handle end of theframe 11. However, this invention is not limited thereto. A racket mayhave an unbranched shaft 30.

Groove Section 111 of Frame 11

FIG. 3 is a diagram of the groove section 111 provided in the frame 11of the present embodiment. FIG. 4 is a diagram of the groove section111′ provided in the frame 11′ of a comparative example. In each ofthese diagrams, of the outer peripheral face 11 a of the frame 11, asolid line indicates the shape (the outer peripheral edge) of the topsurface 112 a of both side sections in the thickness direction (see FIG.2A) as seen from the thickness direction. Another solid line indicatesthe shape (the inner peripheral edge) of the inner peripheral face 11 bof the frame 11 as seen from the thickness direction. A dotted lineindicates the shape of the bottom surface 111 a of the groove section asseen from the thickness direction.

The width of the frame 11 of the present embodiment (the thickness fromthe inner peripheral edge to the outer peripheral edge) is substantiallyuniform through the entire circumference of the frame 11. Of the outerperipheral face 11 a of the frame 11, in other portions in thelongitudinal direction except the handle end, that is, in other portionsexcept the portions connecting to the shaft 30 and the portion facingthe opening 31, the groove section 111 is provided continuously alongthe circumferential direction of the frame 11; the groove section 111 isa section on which the string 12 passing through the string holes 13 tothe outside of the frame 11 is folded back. The depth of the groovesection 111 differs in place. Note that the depth of the groove section111 means the length from the bottom surface 111 a of the groove sectionto the outer peripheral edge in the direction perpendicular to thebottom surface 111 a. The groove section 111 is substantially symmetricabout the line along the longitudinal direction passing the center ofthe frame 11 in the width direction, and is substantially symmetricabout the line along the width direction passing the center of the frame11 in the longitudinal direction.

The depth of the groove section 111 will be described concretely below.The portion of the groove section 111 of the frame 11 provided on thetip end (hereinafter referred to as “the top”) in the longitudinaldirection, in other words, the groove section 111 within a range inwhich longitudinal strings 121 are folded back, gradually becomesshallower in depth as it goes from the center to the outside (left orright) in the width direction. For example, in the top of the frame 11,the depth d1 of the groove section 111 at the center P1 in the widthdirection is deepest. The depth d4 of the groove section 111 at thepoint P4 outside (to the right of) the center P1 in the width directionis shallower than the depth d1 at the center P1. The depth d3 of thegroove section 111 at the point P3 outside the point P4 in the widthdirection is further shallower than the depth d4 at the point P4(d1>d4>d3).

Similarly, The portions of the groove section 111 of the frame 11provided on the ends (hereinafter referred to as “sides”) in the widthdirection, in other words, the portions of the groove section 111 withina range in which transverse strings 122 are folded back, graduallybecome shallower in depth as they go from the center to the outside (thetip end or the handle end) in the longitudinal direction. For example,in the right side of the frame 11, the depth d2 of the groove section111 at the longitudinal center P2 is deepest. The depth d5 of the groovesection 111 at the point P5 outside (to the tip end of) the center P2 inthe longitudinal direction is shallower than the depth d2 at the centerP2. The depth d3 of the groove section 111 at the point P3longitudinally outside the point P5 is further shallower than the depthd5 at the point P5 (d2>d5>d3). It should be noted that, in the presentembodiment, the depth d1 of the groove section 111 of the top of theframe 11 at the center P1 in the width direction is equal to the depthd2 of the groove section 111 of the sides of the frame 11 at thelongitudinal center P2.

On the other hand, in the frame 11′ of the comparative example shown inFIG. 4, unlike the frame 11 of the present embodiment, the groovesection 111′ is uniform in depth. Specifically, in the top of the frame11, the depth d1 of the groove section 111 at the center P1 in the widthdirection is equal to the depth d3 of the groove section 111 at theouter end P3 in the width direction. Also, in the right side of theframe 11, the depth d2 of the groove section 111 at the longitudinalcenter P2 is equal to the depth d3 of the groove section 111 at thelongitudinal outer end P3 (d1=d2=d3). In the comparative example, theelements except for the depth of the groove section 111′ of the frame11′, such as the range in which the groove section 111′ is formed andthe shape of the outer peripheral edge and the inner peripheral edge,are the same as those of the frame 11 of the present embodiment.

The sweet spot, which has a high rebound performance of a ball andvibrates less when hitting a ball, can be enlarged by increasing thelengths of the longitudinal strings 121 and the transverse strings 122and by reducing the difference between the lengths of the longitudinalstrings 121 arranged in the width direction and/or the differencebetween the lengths of the transverse strings 122 arranged in thelongitudinal direction. As shown in FIG. 4, the length (La) of onelongitudinal string 121 means the length in the longitudinal directionfrom the point on the handle-end side at which the longitudinal string121 is folded back to the point on the tip-end side at which thelongitudinal string 121 is folded back (to the bottom surface 111 a ofgroove section; more precisely, to the bottom surface of the groovesection 44 in the base section 42 of the grommet 40). Also, the length(Lb) of one transverse string 122 means the length in the widthdirection from the point on the left side at which the transverse string122 is folded back to the point on the right side at which thetransverse string 122 is folded back. Therefore, in a case where theouter peripheral edge has an elliptical (annular) shape as seen from thethickness direction as in the frame 11 of the present embodiment and theframe 11′ of the comparative example, if the groove section 111 hasconstant depth as in the frame 11′ of the comparative example, thestring 12 is folded back along the bottom surface 111 a′ of the groovesection, which is greatly curved in the same manner as the outerperipheral edge of the frame 11′. Thus, the longitudinal strings 121passing both ends of the frame 11′ in the width direction are shorter inlength than the longitudinal strings 121 passing the center, and thetransverse strings 122 passing both ends of the frame 11′ in thelongitudinal direction are shorter in length than the transverse strings122 passing the center. Therefore, in the frame 11′ of the comparativeexample, only the center serves as the sweet spot, whose area is notsufficiently large.

As opposed thereto, in the frame 11 of the present embodiment, on theouter peripheral face 11 a of the top of the frame 11, compared to theportion of the groove section 111 at a certain position (e.g. P1) in thewidth direction, the portion of the groove section 111 is smaller orless in depth at a position (e.g. P3) outside of the certain position(e.g. P1) in the width direction. Therefore, in the present embodiment,the groove section 111 of the top of the frame 11 extending in the widthdirection has a lower degree of curving than the outer peripheral edgeand the inner peripheral edge of the top of the elliptical frame 11.This groove section 111 also has a lower degree of curving than thegroove section 111′ of the top of the frame 11′ of the comparativeexample extending in the width direction. That is, the bottom surface111 a of the groove section provided in the top of the frame 11 of thepresent embodiment is an arc-shaped surface the shape of which is closerto straight line.

The string 12 is folded back on the groove section 111 provided on theouter peripheral face 11 a of the frame 11 through the base section 42of the grommet 40, and the base section 42 of the grommet is uniform inthickness. Thus, on the outer peripheral face 11 a of the top of theframe 11, compared to a certain position (e.g. P1, corresponding to thefirst position) in the width direction (corresponding to the firstdirection), the depth of the position where the string 12 is folded backis smaller or less at a position (e.g. P3) outside of the certainposition (P1) in the width direction. Not that “the depth at theposition where the string 12 is folded back” when stringing the string12 through the grommet 40 means the distance from the bottom surface ofthe base section 42 of the grommet 40 to the outer peripheral edge inthe direction perpendicular to the bottom surface.

Similarly, on the outer peripheral face 11 a of the sides of the frame11, compared to the portion of the groove section 111 at a certainposition (e.g. P2) in the longitudinal direction, the portion of thegroove section 111 is smaller or less in depth at a position (e.g. P3)outside of the certain position (e.g. P2) in the longitudinal direction.Therefore, in the present embodiment, the groove section 111 of thesides of the frame 11 extending in the longitudinal direction has alower degree of curving than the outer peripheral edge and the innerperipheral edge of the sides of the elliptical frame 11. This groovesection 111 also has a lower degree of curving than the groove section111′ of the sides of the frame 11′ of the comparative example extendingin the longitudinal direction. The bottom surface 111 a of the groovesection provided in the sides of the frame 11 of the present embodimentis an arc-shaped surface the shape of which is closer to straight line.That is, on the outer peripheral face 11 a of the sides of the frame 11,compared to the depth of the position where the string 12 is folded backat a certain position (e.g. P2, corresponding to the first position) inthe longitudinal direction (corresponding to the first direction), thedepth of the position where the string 12 is folded back at a position(e.g. P3, corresponding to the second position) outside of the certainposition (e.g. P2) in the longitudinal direction is smaller or less.

Therefore, in the top of the frame 11 of the present embodiment, theportions of the bottom surface 111 a of the groove section which arelocated on the outer ends in the width direction (the positions at whichthe longitudinal strings 121 are folded back) are located closer to theouter peripheral edge compared to the frame 11′ of the comparativeexample. Similarly, in the sides of the frame 11 of the presentembodiment, the portions of the bottom surface 111 a of the groovesection which are located on the longitudinal outer ends (the positionsat which the transverse strings 122 are folded back) are located closerto the outer peripheral edge compared to the frame 11′ of thecomparative example. Therefore, in the frame 11 of the presentembodiment, the longitudinal strings 121 passing the outer ends of theframe 11 in the width direction can be longer than in the frame 11′ ofthe comparative example. Also, in the frame 11 of the presentembodiment, compared to the frame 11′ of the comparative example, it ispossible to reduce the difference in length between the longitudinalstrings 121 passing the outer ends of the frame 11 in the widthdirection and the longitudinal strings 121 passing the center. Also, thetransverse strings 122 passing the outer ends of the frame 11 in thelongitudinal direction can be longer. And, it is possible to reduce thedifference in length between the transverse strings 122 passing theouter ends of the frame 11 in the longitudinal direction and thetransverse strings 122 passing the center. Therefore, even when hittinga ball with a region far from the center of the frame 11, the differenceis small from when hitting a ball with the central region. This improvesthe rebound performance of a ball and allows the racket to vibrate lesswhen hitting a ball (achieves the same feel). That is, sweet spot can beenlarged.

As a concrete example, consider the following case: the depth of thegroove section 111′ of the frame 11′ in the comparative example is 3 mm(a constant); the depth of the groove section 111 of both sides of theframe 11 in the present embodiment at the longitudinal center (e.g. P2in FIG. 3) is 6 mm; the depth of the groove section 111 at thelongitudinal outer end (e.g. P3 in FIG. 3) is 1 mm; and the depth of thegroove section 111 becomes gradually shallower from the center towardthe outer end in the longitudinal direction. In the frame 11′ of thecomparative example of this case, when the length of the transversestring (the 10th transverse string 122(10) from the tip end) passing thecenter of the frame 11′ in the longitudinal direction is defined as“100%”, the length of the transverse string (the third transverse string122(3) from the tip end) passing the tip-end portion of the frame 11′ inthe longitudinal direction becomes “85%”. As opposed thereto, in theframe 11 of the present embodiment, when the length of the transversestring 122(10) passing the center of the frame 11 in the longitudinaldirection is defined as “100%”, the length of the transverse string122(3) passing the tip-end portion of the frame 11 in the longitudinaldirection becomes “89%”. From this, it will be seen that, compared tothe frame 11′ of the comparative example, the frame 11 of the presentembodiment makes it possible to reduce the difference between thelengths of the transverse strings 122 arranged in the longitudinaldirection.

Further, with the racket 1 of the present embodiment, the sweet spot canbe enlarged without changing the elliptical shape of the frame 11. Theframe 11 of the present embodiment has an accurate elliptical shape.However, the invention is not limited thereto. For example, the frame 11may be substantially elliptical, may have perfect circular shape, or mayhave a shape close to rectangle. However, compared to a frame whoseshape is close to rectangle, a frame having an elliptical or perfectcircular shape obtains more the effect caused by making the lengths ofthe longitudinal strings 121 close to each other or by making thelengths of the transverse strings 122 close to each other. Also, a framehaving an elliptical or perfect circular shape can reduce air resistancecaused when swinging the racket.

The difference between the lengths of the longitudinal strings 121arranged in the width direction and/or the difference between thelengths of the transverse strings 122 arranged in the longitudinaldirection are reduced across wide range as the depth of the groovesection 111 of the top of the frame 11 at the center in the widthdirection is increased and/or as the depth of the groove section 111 ofthe sides of the frame 11 at the center in the longitudinal direction isincreased. However, if the groove section 111 is considerably deep indepth, the thickness from the inner peripheral face 11 b (innerperipheral edge) of the frame 11 to the bottom surface 111 a of thegroove section becomes small, which results in deterioration of thestrength of the frame 11. Therefore, it is preferable that the depth ofthe groove section 111 is decided in the light of this fact.

In the racket 1 of the present embodiment, the groove section 111 isprovided on the outer peripheral face 11 a of the frame 11 and is notprovided on the inner peripheral face 11 b. The lengths of thelongitudinal strings 121 and the transverse strings 122, which affectsenlargement of the sweet spot, are a distance in the longitudinaldirection or in the width direction from one-end side point of thefolding back of the string 12 to other-end side point of the foldingback of the string 12. So, even if the groove section is provided on theinner peripheral face 11 b of the frame 11, the groove section does notcontribute enlargement of the sweet spot. Further, providing the groovesection 111 on the outer peripheral face 11 a of the frame 11 makes amanufacturing process of the frame 11 easier than providing a groovesection on the inner peripheral face 11 b (for example, a mold forshaping the frame 11 into an ellipse is easy to be manufactured).Furthermore, compared to the case in which the groove section 111 isprovided on the inner peripheral face 11 b of the frame 11, providingthe groove section 111 on the outer peripheral face 11 a makes itpossible to reduce stress concentration when hitting a ball, and canincrease the strength of the frame 11.

In the racket 1 of the present embodiment, in the top of the frame 11,the portions of the groove section 111 at the outer ends in the widthdirection are shallower than the portion of the groove section 111 atthe center; the thickness and the strength from the inner peripheraledge to the bottom surface 111 a of the groove section increase.Similarly, in the sides of the frame 11, the portions of the groovesection 111 at the outer ends in the longitudinal direction areshallower than the portion of the groove section 111 at the center; thethickness and the strength from the inner peripheral edge to the bottomsurface 111 a of the groove section increase. Thus, when hitting a ball,the frame 11 is likely to be curved inwardly in the longitudinaldirection and in the width direction (the side which the ball hits isdefined as the inside). That is, the frame 11 is likely to deform so asto rap the ball. This makes it possible to increase the contact areabetween the ball and the hitting surface (string 12), which results inimprovement of control and spin.

If the frame 11 has an elliptical (annular) shape, when the groovesection 111′ is uniform in depth as in the frame 11′ of the comparativeexample, the longitudinal strings 121 become shorter as they passthrough either outer portion of the frame 11 in the width direction, andthe transverse strings 122 become shorter as they pass through eitherouter portion of the frame 11 in the longitudinal direction. Thus, inthe racket 1 of the present embodiment, the portions of the groovesection 111 which are located on the outer peripheral face 11 a of theframe 11, that is, the portions where the string 12 is folded back, areshallower in depth as they go outwards from the center in thelongitudinal direction or the width direction of the frame 11. That is,in the top of the frame 11, the portion of the groove section 111 (thepositions at which the string 12 is folded back) becomes shallower indepth as it goes outwards in the width direction. Also, in side of theframe 11, the portions of the groove section 111 (the positions at whichthe string 12 is folded back) become shallower in depth as they gooutwards in the longitudinal direction. This makes it possible to reducethe difference between the lengths of longitudinal strings 121 arrangedin the width direction and/or the difference between the lengths oftransverse strings 122 arranged in the longitudinal direction whileincreasing the lengths of the longitudinal strings 121 and thetransverse strings 122. This makes it possible to reduce the differencescaused by different hitting positions in the frame 11 of the reboundperformance of a ball or the feel, which results in enlarging the sweetspot.

Further, it is preferable that the groove section 111 varies graduallyin depth without varying in steps in depth of the groove section 111, inother words, without steps in the bottom surface 111 a of the groovesection. This makes it possible to prevent stress when hitting a ballfrom concentrating into steps provided in the bottom surface 111 a ofthe groove section and to increase the strength of the frame 11. Thisalso makes it possible to prevent change of the feel produced only whena ball hits a corresponding portion within the frame 11 to the stepsprovided in the bottom surface 111 a of the groove section.

Modified Example

In the abovementioned embodiment, the base section 42 of the grommet 40is uniform in thickness. However, the invention is not limited thereto.For example, on the outer peripheral face 11 a of the top of the frame11, the portions of the base section 42 of the grommet 40 provided inthe outside in the width direction may be thicker than the portions ofthe base section 42 of the grommet 40 provided in the center in thewidth direction. Similarly, on the outer peripheral face 11 a of thesides of the frame 11, the portions of the base section 42 of thegrommet 40 provided in the outside in the longitudinal direction may bethicker in thickness than the portions of the base section 42 of thegrommet 40 provided in the center in the longitudinal direction.

Compared to the case in which only the depth of the groove section 111of the frame 11 varies, this makes it possible to further reduce thedifference between the lengths of longitudinal strings 121 arranged inthe width direction and also to further reduce the difference betweenthe lengths of transverse strings 122 arranged in the longitudinaldirection. Further, a thick portion of the base section 42 of thegrommet 40 is located far from the frame 11 which is highly rigid, sothat elastic deformation of the grommet 40 easily to occur. Thus, when aball impacts the hitting surface, if an force toward the inside of theframe 11 is produced at the positions where the string 12 is foldedback, the force can be absorbed by the thick grommet 40 (the basesection 42). Thus, though the portions of the string passing in theouter ends in the width direction or in the longitudinal of the frame 11are slightly shorter than the portions of the string passing in thecenter, the thick grommet 40 (the base section 42) has a high vibrationdamping capacity. Therefore, even when a player hits a ball with aregion far from the center of the frame 11, the player can experiencethe feel nearly as much as when hitting a ball with a region of thecenter. That is, the sweet spot can be enlarged.

It should be noted that, if the base section 42 of the grommet 40 variesin thickness, the groove section 111 of the frame 11 may be uniform indepth. In other words, adjusting the thickness of the base section 42 ofthe grommet 40 may achieve the followings: on the outer peripheral face11 a of the frame 11, compared to a certain position in the longitudinaldirection, the depth of the position where the string 12 is folded backis shallower at a position outside of the certain position in thelongitudinal direction; on the outer peripheral face 11 a of the frame11, compared to a certain position in the width direction, the depth ofthe position where the string 12 is folded back is shallower at aposition outside of the certain position in the width direction.

FIG. 5 is a diagram of the frame 11 and the groove section 111 of themodified example. The frame 1 of the racket 1 in the abovementionedembodiment has an elliptical shape. However, the invention is notlimited thereto. For example, as shown in FIG. 5, the frame 1 may havesubstantially an elliptical shape in which the tip end and the handleend in the longitudinal direction have a lower degree of curving thanthose of an ellipse.

Further, in the racket 1 of the abovementioned embodiment, the depth(d1) of the point (P1 in FIG. 3) of the groove section 111 of the top ofthe frame 11 at the center in the width direction is equal to the depth(d2) of the point (P2 in FIG. 3) of the groove section 111 of the sidesof the frame 11 at the center in the longitudinal direction. However,the invention is not limited thereto. The groove section 111 may vary indepth. As mentioned above, the difference between the lengths oflongitudinal strings 121 arranged in the width direction and/or thedifference between the lengths of transverse strings 122 arranged in thelongitudinal direction can be reduced as the depth of the groove section111 of the top of the frame 11 at the center in the width direction isincreased and/or as the depth of the groove section 111 of the sides ofthe frame 11 at the center in the longitudinal direction is increased.This reduces the thickness of the frame 11 from the inner peripheralface 11 a to the bottom surface 111 a of the groove section, whichresults in deterioration of the strength of the frame 11. However, ifthe frame 11 is substantially in an elliptical shape (including aperfect elliptical shape), the frame 11 is strong and difficult todeform because the top of the frame 11 is smaller in size than thesides. Further, since longitudinal strings 121 are longer thantransverse strings 122, reducing the difference between the lengths oflongitudinal strings 121 arranged in the width direction is moreeffective than reducing the difference between the lengths of transversestrings 122 arranged in the longitudinal direction. That is, it ispossible to further enlarge the sweet spot; also this allows the racketto rebound a ball more and to vibrate less.

As shown in FIG. 6, it is preferable that, in the frame 11 havingsubstantially an elliptical shape, the depth (D+α) of the point (P1 inFIG. 5) of the groove section 111 of the outer peripheral face 11 a ofthe top (the end in the longitudinal direction) at the center in thewidth direction be deeper than the depth (D) of the point (P2 in FIG. 5)of the groove section 111 of the outer peripheral face 11 a of eitherside (either end in the width direction) at the center in thelongitudinal direction. That is, it is preferable the depth at theposition where the string 12 is folded back on the outer peripheral face11 a of the top of the frame 11 at the center in the width direction isdeeper than the depth at the position where the string 12 is folded backon the outer peripheral face 11 a of the sides of the frame 11 at thecenter in the longitudinal direction. As a result, the sweet spot can befurther enlarged while preventing the strength deterioration of theframe 11. This also allows the racket to rebound a ball more and tovibrate less.

In the racket 1 of the abovementioned embodiment, all portions of thegroove section 111 located in the top and both sides of the frame 11become shallower in depth as they go from the center to the outer ends.However, the invention is not limited thereto. For example, at least oneportion of the groove section 111 located in the top and both sides ofthe frame 11 may be different in depth. Further, in the racket 1 of theabovementioned embodiment, the groove section 111 is not provided in thehandle end (the portion facing the opening 30) of the frame 11 in thelongitudinal direction. However, the invention is not limited thereto.For example, the groove section 111 may be provided in the handle endand the groove section 111 may be shallower in depth as it goes from thecenter in the width direction to the outer ends. Furthermore, in theracket 1 of the abovementioned embodiment, the groove section 111 variesgradually in depth. However, the invention is not limited thereto. Forexample, the groove section 111 may vary in depth by providing steps inthe bottom surface 111 a of the groove section.

Other Embodiments

It should be noted that the aforementioned embodiment is forfacilitating understanding of the invention, and are not limiting of theinvention, and are not to be interpreted as limiting the invention. Theinvention can of course be altered and improved without departing fromthe gist thereof, and equivalents are intended to be embraced therein.

For example, in the abovementioned embodiment, a tennis racket is takenas an example of a racket associated with the invention. However, theinvention is not limited thereto. For example, the invention may beapplied to a squash racket, a badminton racket and the like. Also, inthe abovementioned embodiment, the racket 1 in the frame 11 of which thestring 12 is strung is taken as an example of a racket associated withthe invention. However, the invention is not limited thereto. A racketin which the string 12 is not strung may also be employed.

1. A racket comprising: a handle; an annular frame having a plurality ofthrough holes, and a groove section provided on an outer peripheral faceof the frame along a circumferential direction of the frame; a shaftconnecting the handle and the frame; and a string that passes throughthe through hole to the outside of the frame, and that is folded back onthe groove section, wherein when either one of a longitudinal directionof the racket and a width direction perpendicular to the longitudinaldirection on a hitting surface of the racket is defined as a firstdirection, a depth of a position at which the string is folded back at asecond position in the first direction is smaller or less than a depthof a position at which the string is folded back at a first position inthe first direction, the first position being located on the outerperipheral face of the frame, the second position being located on theouter peripheral face of the frame and outside of the first position inthe first direction.
 2. A racket according to claim 1, wherein theposition at which the string is folded back on the outer peripheral faceof the frame becomes shallower in depth as it goes from a center of theframe to an end in the first direction.
 3. A racket according to claim1, wherein the frame has substantially an elliptical shape, and aposition in an end in the longitudinal direction on the outer peripheralface of the frame at which the string is folded back at a center in thewidth direction is deeper in depth than a position in an end in thewidth direction on the outer peripheral face of the frame at which thestring is folded back at a center in the longitudinal direction.